1. Field of the Invention
This invention relates to the recovery of iron, flux and carbon units contained in blast furnace flue dust and sludge and similar steel plant revert materials by mixing such materials with water and a cement binder, casting the mixture, crushing the material and recycling it to a blast furnace.
2. Description of Prior Art
Blast furnace and steelmaking furnace off-gases are cleaned by a dry process and a wet process (water washing), producing both dry dust and wet sludges.
Iron-, flux- and carbon-bearing waste materials ("reverts") generated in the production of iron and steel historically have been recycled back through the blast furnace after agglomeration by sintering. However, with the decreasing number of operating sinter plants, landfilling these revert materials became necessary. Steel plant flue dust and sludges were taken by truck to landfills where they were disposed of. With newly imposed landfill requirements, increasing transportation and related landfill costs, and heightened environmental awareness, a need has been created for alternative agglomeration processes to produce revert agglomerates with properties suitable for recycling back through the blast furnace. Thus, landfill costs could be decreased and valuable iron, carbon and fluxing agent units contained in the reverts could be utilized.
Early attempts at recycling steel plant by-product dusts for use in the blast furnace were directed to the forming of briquettes in a rotary-briquetting machine to form an agglomerate bonded by Portland cement, hydrated lime, and molasses. In this rotary-briquetting operation, molasses was used to provide sufficient early strength in the briquettes to permit stockpiling the agglomerates until their final strength was developed through hydration and curing of the cement. These agglomerates had adequate physical strength (ISO tumbler strength), but a lower than desired hot strength [strength after reduction under conditions simulating those present in the blast furnace, e.g. the strength index determined after testing in the U.S.Steel simulated blast furnace (SBF) test]. The use of molasses in agglomerates intended for the blast furnace adversely affected the hot strength of the agglomerates and, moreover, was later found to create environmental problems, which prompted discontinuance of this approach.
U.S. patent application Ser. No. 08/107,270 discloses recovery of blast furnace flue dust, steelmaking slag and finishing mill roll scale by agglomerating such materials with water, cement and bentonite binder in which the bentonite accounts for about 25-30% by weight of binder, and forming briquettes of such mixtures. The maximum water content for forming briquettes with useful cold strength and high temperature reduction strength is about 10%.
The sludge resulting from the wet cleaning of off-gases contains considerable water which, with the water necessary for hydration of a cement or cement/bentonite binder, often substantially exceeds 10%. Drying of the sludge is expensive and is to be avoided if possible. Moreover, the use of bentonite in such briquettes also is expensive.